"""
难度：简单
泰波那契序列 Tn 定义如下： 
T0 = 0, T1 = 1, T2 = 1, 且在 n >= 0 的条件下 Tn+3 = Tn + Tn+1 + Tn+2
给你整数 n，请返回第 n 个泰波那契数 Tn 的值。

示例 1：
输入：n = 4
输出：4
解释：
T_3 = 0 + 1 + 1 = 2
T_4 = 1 + 1 + 2 = 4
示例 2：
输入：n = 25
输出：1389537
提示：
0 <= n <= 37
答案保证是一个 32 位整数，即 answer <= 2^31 - 1
"""
class Solution:
    # 动态规划
    def tribonacci(self, n: int) -> int:
        if n == 0:
            return 0
        if n <= 2:
            return 1
        p = 0
        q = r = 1
        for i in range(3,n+1):
            s = p + q + r
            p, q, r = q, r, s
        return s



    def tribonacci(self, n: int) -> int:
        nums = [0] * 38
        nums[1] = nums[2] = 1
        for i in range(3, n):
            nums[i] = nums[i-1] + nums[i-2] + nums[i-3]
        return nums[n]